Wednesday, 18 December, 2002, 18:59 GMT
The adult brain appears much more able to rewire itself in response to stimuli from the outside world than previously thought, scientists have discovered.
A US team has used a groundbreaking technique to study activity in the brains of living mice.
Their work has allowed them to observe adult brain cells forming new connections.
The finding contradicts the widely held view that the structure of adult brains is fixed, and therefore the capacity for recovering from injury is limited.
It may also help scientists to gain a greater understanding of the processes underpinning learning and memory.
And it may even lead to new ways to treat brain injuries and mental retardation.
The researchers, from Cold Spring Harbor Laboratory, used state-of-the-art technology to show that connections between brain cells - known as synapses - form and dissolve in the adult brain as the mice take in sensory information.
The scientists used genetic engineering techniques to create mice with brain cells that produced a fluorescent green protein.
Then, they placed a small window over the part of the brain they wanted to study - the barrel cortex, a region associated with receiving information the mice gather with their whiskers.
Every 24 hours for eight days and less frequently for the rest of a month, they checked to see which neurons sent out and retracted spines to form and eliminate connections with other neurons.
To see if the changing connections were influenced by sensory input they cut every other whisker on the mice, creating a chessboard pattern in which each cut whisker was surrounded by uncut whiskers, and then let the mice explore an unfamiliar environment.
They found that the total number of synapses stayed relatively constant but the individual connections often changed.
Some stuck around for only a few days and others, generally the thicker ones, stayed for the duration of the experiment.
Significantly, connections formed and dissolved much more rapidly after the animals' whiskers were cut and they were placed in the novel environment.
This suggests that the synapses changed according to new sensory input.
The researchers, led by neurobiologist Professor Karel Svoboda, think it possible that brain cells put out connections to each other on a random basis.
Those synaptic connections that prove to be useful are reinforced and grow thicker, while those that have little use wither away.
Their next move will be to test whether synapses that are used more are the ones that grow thicker.
Professor Svoboda told BBC News Online that he was quite surprised by the ability of the adult brain to make new connections.
"The brain operates with circuitry that is constantly changing in response to new demands.
"However, we think that the plasticity in the adult is quite different, and much more limited, than that observed in the developing brain.
"Whereas in the developing brain the large-scale structure of neurons changes in response to experience, in the adult brain these structural changes are primarily local."
Dr Paul Adams, a neurobiologist at the State University of New York, Stony Brook said: "If a few years ago you could have imagined in your wildest dreams the experiment you wanted to do, it would be this one.
"To show that, in a relatively short period of time, synapses grow in an adult brain."
The research is published in the magazine Nature.
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